a) Field of the Invention
The invention is directed to a method for encoding machine-readable measurement scales. Encoded scales have many uses in measurement technology. The method can be used in geodetic instrument engineering to produce a digital and consequently machine-readable level indicator. Other possible applications are in angle and distance measurement.
b) Background Art
A measurement scale must have suitable graduations which can be read visually or mechanically in order for the position of a mark on the scale to be detected. For visual reading, graduations are commonly provided with written numerals, whereas different bar codes are used for mechanical reading. In bar codes, the information content can be encoded by color, bars or spaces, by the dimension of the bars and spaces or by the color and dimension. The code can be arranged on the scale in the longitudinal direction (DE 3739664, DE 3424806), transverse direction (EP 0290140, JP 60-25413) or in both the longitudinal and transverse direction.
The code can be produced by using selected generator polynomials of pseudostochastic bit sequences. A prominent characteristic of these sequences consists in that every possible number can only occur once within their periodicity.
The information content of a pseudostochastic code can be increased by converting this code by means of a biphase code as is indicated in DE 3739664. In so doing, for example, an interval on the scale of length l with uniform color is allocated to every 1-bit and two intervals, each having a length 1/2, with dark and light color are assigned to every 0 bit. At every bit border, the color changes from light to dark or from dark to light.
The length of scale L.sub.M is given by the following equation: EQU L.sub.M =(2.sup.k +k-2)L.sub.B ( 1),
where k is the number of bits required to encode a number and L.sub.B is the length of the individual code element.
When a scale encoded in this way is read by an optical system, the largest and smallest possible scale division or measurement distance, among other things, is determined by the focal length and by the resolution capability of the receiver. The greatest measurement distance S.sub.max is given by the following equation: EQU S.sub.max =(f.multidot.L.sub.B)/(2.multidot.ov.multidot.L.sub.P)(2),
where L.sub.P represents the magnitude of the receiver element, f is the focal length, and ov is the oversampling rate.
The limiting case for meeting the requirements of the sampling theorem occurs at ov=1.
The smallest measurement distance S.sub.min is given by the following equation: EQU S.sub.min =(f.multidot.k.multidot.L.sub.B)/(N.sub.P .multidot.L.sub.P)(3),
where N.sub.P is the number of receiver elements.
The ratio of maximum measurement distance to minimum measurement distance is given by the following equation: EQU V.sub.s =N.sub.P /(k.multidot.2.multidot.ov) (4).
In order to increase this ratio, it would be necessary to reduce the number of bits or increase the number of receiver elements. According to equation (1), a reduction in the number of bits would result in a reduction in the length of the scale. An increased number of receiver elements results in an increase in the optical image field to be illuminated, which would require larger optical elements and stricter quality requirements. It is known from DE 3424806 that the ratio of maximum measurement distance to minimum measurement distance is brought about by a violation of the sampling theorem. The bit lengths are selected so as to be so small enough to result in a sufficiently small measurement distance. Since these small bars can no longer be resolved in a definite manner at the maximum measurement distance, code reading is carried out by means of an integral comparison operation between the scale imaged on the receiver and a stock of scale images stored in the evaluating device as a function of the measurement distance and section of the scale. This method is disadvantageous in that it requires prior information concerning the measurement distance obtained from the position of the focussing drive and the section of the scale which must be imaged must be greater than the actual length of a code.
A method for generating pseudostochastic random sequences for encoded measurement scales is described in EP 0441963. The pseudostochastic coding is effected by a CCD sensor or the like with a fixed imaging scale and is accordingly not suitable for application to geodetic devices.
WO 84/01027 describes a pseudostochastic random sequence as code for a scale with the object of achieving resolutions which are smaller than a coding element when carrying out measurements. For this purpose, a plurality of receiver elements of the receiving sensor are associated in a fixed manner with a coding element on the scale. This is also only a matter of interpolation with constant imaging ratios. The object of the invention is to provide a method for encoding a machine-readable scale so that a scale which has been encoded according to this method can be used for different measurement distances.